GB2168804A

GB2168804A - Apparatus for measuring transmission characteristics of an optical fibre

Info

Publication number: GB2168804A
Application number: GB08527606A
Authority: GB
Inventors: Yoshihiro Takiguchi
Original assignee: Hamamatsu Photonics KK
Current assignee: Hamamatsu Photonics KK
Priority date: 1984-11-09
Filing date: 1985-11-08
Publication date: 1986-06-25
Also published as: JPH0354776B2; US4767207A; JPS61116634A; GB2168804B; GB8527606D0

Description

GB 2 168 804 A 1

SPECIFICATION

Apparatus for measuring transmission characteris tics of an optical fibre The present invention relates to an apparatus for measuring the transmission characteristics of an optical fibre.

Apparatus is commercially available for measur 10 ing the transmission characteristics of an optical fibre, especially distortions in the waveform of light being transmitted through the fibre. In an ex ample of a conventional apparatus a laser diode is driven by a laser diode drive unit to emit pulses of light. These pulses of light are incident upon one end of an optical fibre under test and the light emerging from the other end of the fibre is con verted in to an electrical signal by a photodiode.

The output of the photodiode is amplified by an 20 amplifier and monitored on an oscilloscope. A rough estimate of the characteristics of the fibre under test can be obtained by observing the wave form on the CRT. If a more accurate determination is needed, the output of the amplifier is digitized 25 by an A-D converter and stored in a memory for future use in analysis.

The maximum time resolution that can be achieved by such a system is on the order of 102 picoseconds and this is not high enough to resolve 30 small waveform distortions. With a view to com pensating for the low time resolution of this sys tern by magnifying waveform distortions, it is recommended that an optical fibre under test has a length of about 1 km. Such a method, however, is 35 incapable of accurate measurement because a dis- 100 tortion that occurs in part of the 1 km long fibre is only picked up after being averaged over the entire length of the 1 km long fibre.

According to this invention an apparatus for 40 measuring light transmission characteristics of an 105 optical fibre comprises converting means including a photoelectron emitting surface for converting light incident upon the photoelectron-emitting surface into an optical 45 output signal which is resolved with respect to time to indicate changes in the luminance of the light received by the photoelectron-emitting sur face with respect to time; generating means for generating short duration 50 light pulses; a reference optical fibre; and, connector means connected to the reference op tical fibre between the generating means and the converting means and arranged to connect an opti 55 cal fibre under test between the generating means and the converting means so that light pulses gen erated by the generating means are applied to both fibres and so that the optical output signal in dicates differences in light transmission character 60 istics between the reference optical fibre and the optical fibre under test.

The present invention provides an apparatus which is capable of accurately measuring a wide variety of characteristics of the optical fibre and which is capable of high resolution. It can also measure transmission characteristics of an optical fibre which is short in length.

Two examples of apparatus in accordance with this invention will now be described and con- 70 trasted with respect to the prior art with reference to the accompanying drawings; in which:- Figure 1 is a block diagram of an apparatus according to a first example oi the present invention; Figure 2 is a block diagram part of a second ex- 75 ample of the present invention; Figures 3 and 4 are schematic diagrams illustrat ing the output of the apparatus shown in Figure 1; Figure 5 is a schematic diagram illustrating the output of the apparatus shown in Figure 2; and, 80 Figure 6 is a block diagram of a conventional ap paratus for measuring transmission characteristics of an optical fibre.

An example of a conventional apparatus is shown in Figure 6, in which a laser diode LD is dri ven by a laser diode drive unit to emit pulses of light. These pulses of light are incident upon one end of an optical fibre under test F and the light emerging from the other end of the fibre is con verted in to an electrical signal by a photodiode 90 PD. The output of PD the photodiode is amplified by an amplifier Amp and monitored on an oscilloscope M. A rough estimate of the characteristics of the fibre under test can be obtained by observing the waveform on the CRT. If a more accurate de- 95 termination is needed, the output of the amplifier is digitized by an A-D converter A/D and stored in a memory for future use in analysis.

The maximum time resolution that can be achieved by such a system is on the order of 102 picoseconds and this is not high enough to resolve small waveform distortions. With a view to compensating for the low time resolution of this systern by magnifying waveform distortions, it is recommended that an optical fibre under test has a length of about 1 km. Such a method, however, is incapable of accurate measurement because a distortion that occurs in part of the 1 krn long fibre is only picked up after being averaged over the entire length of the 1 km long fibre.

110 Figure 1 is a block diagram of one embodiment of the apparatus of the present invention for meas uring the transmission characteristics of an optical fibre. With a view to attaining a high time resolu tion, the apparatus of the present invention uses a 115 streaking means that converts a change in the intensity of light incident upon a photoelectron-emitting surface or a photocathode into a pattern of fluorescent luminance on a fluorescent screen having a time axis in sweeping direction. The streak- 120 ing means includes a streak tube 7 and a streak tube drive unit 15 that applies an operating voltage and sweep voltage to the streak tube 7. The use of the streak tube 7 ensures a time resolution of up to 2 picoseconds. For example, an apparatus numbered as "C1370-01" or "C1587" consisting of a streak tube and streak tube driver manufactured by Hamamatsu Photonics K.K., which is commercially available, is suitable for achieving this time resolution. A light source 10 for generating short light 130 pulses includes a GaAlAs or InGaAsP laser diode 2 GB 2 168 804 A 2 12. This laser diode is driven by a laser diode driver unit 11 to generate laser pulses.

The laser diode 12 used in the embodiment shown has a wavelength range of 800 nm to 1.5 5 pm. Part of the laser pulses is monitored by a monitor diode 14 and the monitor output is fed back to the laser diode drive unit 11 so that the laser diode 12 will provide a constant output of light pulses. In order to match the speed of the 10 laser diode 12, the monitor diode 14 is made of a GaAs or GaAlAs photodiode. The laser diode drive unit 11 is controlled for start and other operating modes by an output from a controller 19 which will be described later in this specification.

15 The output from the laser diode 12 is also 80 passed through a mode scrambler 13 to be con nected to a laser beam splitter unit 9 composed of glass beam splitters. The mode scrambler 13 con sists of several turns of an optical fiber would 20 around a mandrel with a diameter of 30 mm. The laser pulses emitted from the laser diode produces spatial distributions of the intensity and wavelength of laser light as a result of laser oscillations. The mode scrambler 13 has the function of provid- 25 ing a uniform intensi ty and wavelength distribution across the entire section of the optical fiber.

One of the laser beams issuing from the beam splitter unit 9 is connected to a reference optical fiber 1 the output end of which is connected to the 30 photoelectron-emitting surface of the streak tube 7 by a connector 4. The other laser beam is connected to an optical fiber 2 under test. The output end of the fiber 2 is also connected to the photoelectron-emitting surface of the streak tube 7 by a 35 connector 3.

In the embodiment shown, the reference optical fiber 1 is a multimode quartz fiber of the graded index type with a length of 1 m having core and cladding diameters of 50 Lrn and 125 Lrn, respec- tively. An example of such multimode optical fiber is G. 501125 - 2510 available from Fujikura, Ltd. This type of optical fiber is extensively used in high-speed optical transmission and is suitable for use as a reference for the testing of general-pur- 45 pose optical fibers.

A single-mode quartz optical fiber such as SM. 6/ 125.30 of Fujikura, Ltd. , which has core and cladding diameters of 6pm and 125 pm, respectively, may be used as a reference for the testing optical fibers for higher-speed transmission. This singlemode fiber must of course be used with a singlemode optical connector.

The reference optical fiber 1 and the optical fiber under test 2 are connected via respective connec- tors 4 and 3 to the streak tube 7 to be aligned with a line perpendicular to the direction in which the photoelectrons (or the electron beam) in the streak tube 7 are deflected and sweep the fluorescent screen.

An imaging device 8 is positioned in front of the fluorescent screen 7a of the streak tube 7 for taking a picture of a streak image. The output of the imaging device 8 is amplified by an amplifier 16 and fed through a monitor drive unit 17 for display on 65 a television monitor 21. The imaging device 8 may be made of a high-sensitivity silicon intensified target (SIT) camera. The output of the amplifier 16 is stored in a memory 18.

The controller 19 is fed from an external circuit 70 with the necessary control information (e.g. the length of a laser pulse and the speed at which the streak tube is swept) for generating signals that will sequence-control or synchronize the respective components of the apparatus of the present inven- 75 tion. The data stored in the memory 18 are analyzed by an analyzer 20 and read out by a suitable output device (not shown).

The apparatus described above with reference to Figure 1 may be used to measure various transmission characteristics of an optical fiber as shown below.

Measuring the length of an optical fiber or the speed at which light propagates through the fiber:

The light source 10 is capable of generating short light pulse s, and by generating pulses with a duration of 10 picoseconds, from this light source, - the length of an optical fiber or the speed at which light propagates through the fiber can be meas- 90 ured. If both the length and material of the reference optical fiber 1 are known, the length of the optical fiber 2 and the delay time can be measured by the apparatus of the present invention.

Suppose that two images emerge on the fluores- 95 cent screen 7a of the streak tube 7 as shown in Figure 3. Image (1) is due to the light issuing from the reference optical fiber 1, and image (2) results from the light coming from the optical fiber 2. If the two optical fibers are made of the same mate- 100 rial and are identical in cross-sectional shape, the difference between the lengths of the two fibers, Ae, is given by the formula C-At, where C is the speed of light traveling through each fiber. If the two fibers are made of different materials, the refractive-index profile of the optical fiber under test or the speed of light propagating through that fiber can be measured with the apparatus of the present invention.

110 Measuring distortion in light transmission -through optical fiber:

The first requirement that should be satisfied for measuring distortion in light transmission through the optical fiber 2 is to provide a reference optical 115 fiber 1 having a cross-sectional shape identical to that of the optical fiber 2. The schematic illustration of the fluorescent screen 7a of the streak tube 7 as shown in Figure 4 shows an image (1) due to the light issuing from the reference optical fiber 120 and an image (2) resulting from the light coming from the optical fiber 2. By comparing the intensity distributions of the two images in the direction of the time axis, any distortion in light transmission through the optical fiber 2 can be measured.

125 This case of measurement assumes that the light source 10 generates light pulses with a duration of picoseconds and that the fluorescent screen 7a has an effective time axis with a length of 300 pi coseconds. Information such as attenuation ratio, 130 time dispersion and distortion in transmission can GB 2 168 804 A 3 be elicited by analyzing the output waveforms.

Figure 2 is a block diagram showing another embodiment of the apparatus of the present invention for measuring the characteristics of an optical fi5 ber. In Figure 2, only the portions that differ from the embodiment in Figure 1 are explicitly depicted. As shown, the second embodiment of the present invention is characterized by a spectrometer 6 inserted between the streak tube 7 and the reference 10 optical fiber 1 and the fiber 2 that are connected to the spectrometer by the connector 4 and the connector 3, respectively. The spectrometer 6 provides as its output a spectrum so that intensity distributions for different wavelengths will be produced on the photoelectron-emitting surface of the streak tube 7 in a direction normal to the direction in which the photoelectros in the streaking means are deflected and sweep the fluorescent screen.

The light source 10 generates the necessary 20 number of light pulses to ensure satisfactory detection of any faint light component in the output spectrum The streak tube drive unit 15 in the streaking means can apply sweep signals to the streak tube 7 synchronized with the repetitive light 25 pulses so overlapping repetitive streak images will be obtained on the fluorescent screen 7a.

The operation of the system in accordance with the second embodiment will hereunder be described with reference to Figure 5.

Measuring transmission characteristics for different wavelengths:

Light spectra obtained by the spectrometer 15 will cause streak images to emerge on the fluores- 35 cent screen 7a as shown in Figure 5. By comparing the image (1) due to the light transmitted through the reference optical fiber 1 with the image (2) resulting from the light transmitted through the optical fiber 2, the characteristics of light transmission through the optical fiber 2 can be compared for different wavelengths. If, as shown in Figure 5, no image of light component at a wavelength X5 emerges, it can be concluded that the cutoff frequency of the optical fiber 2 lies between wave- 45 lengths X4 and X5.

Measuring raman scattering occurring within an optical fiber:

The Raman scattering effect will occur within 50 every type of optical fiber because it is caused by the interaction between the constituent molecules of the optical fiber and the light that is incident upon the fiber. The Raman light is generated in a quantity proportional to the intensity of incident light and is affected by the fiber's transmission loss. The wavelength of the Raman light is approximately 50 nm longer than the wavelength of the incident light, so that Raman scattering can be quantitatively evaluated by measuring the intensity 60 and temporal distributions of the Raman spectra obtained by the spectrometer 6.

The two embodiments. described above may be modified in various ways that will not depart from the scope and spirit of the present invention as set forth in the appended claims. In each of the em- bodiments shown above, the imaging device 8 is made of a SIT camera but satisfactory results can be obtained by using a solidstate imaging device such as two-dimensional CCID camera.

70 As described above, the apparatus of the present invention is so constructed that light pulses trans mitted through each of the reference optical fibre and the optical fibre under test will be detected by the streak tube. This arrangement enables the measurement of the transmission characteristics of short optical fibres, which has been impossible with the prior art apparatus. Measurement of opti cal transmission characteristics for different wave lengths has the advantage in that many 80 parameters (e.g. wavelength, intensity, delay time and wavelengthdispersion in the fibre) can be measured simultaneously for a plurality of wavelengths.

Claims

85 CLAIMS

1. An apparatus for measuring light transmission characteristics of an optical fibre comprising:

converting means including a photoelectron- 90 emitting surface for converting light incident upon the photoelectron- emitting surface into an optical output signal which is resolved with respect to time to indicate changes in the luminance of the light received by the photoelectron-emitting sur- 95 face with respect to time; generating means for generating short duration light pulses; a reference optical fibre; and, connector means connected to the reference op- 100 tical fibre between the generating means and the converting means and arranged to connect an optical fibre under test between the generating means and the converting means so that light pulses generated by the generating means are applied to 105 both fibres and so that the optical output signal indicates differences in light transmission characteristics between the reference optical fibre and the optical fibre under test.

2. An apparatus according to claim 1, wherein 110 the optical output signal indicates temporal dispersion and waveform distortions in the light transmission characteristics of the optical fibre under test with respect to the reference optical fibre.

3. An apparatus according to claim 1, which 115 also includes a spectrometer connected between the ends of the optical fibre under test and the reference fibre and the photoelectron- emitting surface of converting means, the spectrometer resolving the light carried by the fibres with respect to wave- 120 length so that the output of the converting means provides a spectrum having an intensity distribution with respect to wavelength.

4. An apparatus according to claim 3, wherein the optical output signal indicates temporal disper- 125 sion and waveform distortion of the optical fibre under test for different wavelengths of the light pulses transmitted through the optical fibre under test.

5. An apparatus according to claim 3, wherein 130 the output optical signal indicates the optical cutoff 4 GB 2 168 804 A 4 of the optical fibre under test.

6. An apparatus according to any one of the preceding claims, whichalso includes an imaging device for producing electrical images of the opti- 5 cal output signal produced by the converting means.

7. An apparatus according to claim 6, further including memory means for storing the electrical images produced by the imaging device.

8. An apparatus according to any one of the preceding claims, wherein the connector means includes a beam splitter located between the generating means and the optical fibres to divide the light pulses emitted by the generating means be- tween the optical fibre under test and the reference fi b re.

9. An apparatus according to any one of the preceding claims, wherein the generating means comprises a laser.

10. An apparatus for measuring light transmission characteristics of an optical fibre substantially as described with reference to the accompanying drawings.

11. An apparatus for measuring light transmis- 25 Sion characteristics of an optical fibre comprising:

means for converting changes in the intensity of light incident upon a photoelectron-emitting surface thereof into an optical output signal indicating time wise changes in the luminance of said light 30, received by said photoelectron-emitting surface; a reference optical fibre having a first end connected to said photoelectron-emitting surface and a second end; an optical fibre under test having a first end con- 35 nected to said photoelectron-emitting surface and a second end; means for generating short light pulses; and connector means connected to said generating means and to said second end of said reference 40 optical fibre and said second end of said optical fibre under test to supply said light pulses to said reference optical fibre and said optical fibre under test such that said optical output signal indicates differences in light transmission characteristics be- tween said reference optical fibre and said optical fibre under test.

Printed in the UK for HIVISO, D8818935, 5/86, 7102. Published by The Patent Office, 25 Southampton Buildings, London, WC2A IAY, from which copies may be obtained.